Switchable color filters which include birefringent materials have been known heretofore to be positioned in front of black and white cathode ray tubes of field sequential color display systems to produce light images in full color. One such display system is disclosed in U.S. Pat. No. 2,638,816 of Stolzer who describes an adapter which receives light emitted from a black and white television set and produces a light image in different colors. The adapter includes passive birefringent sheets which develop light of different colors. A half-wave optical retardation device operates as a light gate which transmits light of the desired color in the proper sequence in synchronism with the frames of image information appearing on the black and white television screen.
One disadvantage inherent in the device of Stolzer is that the colors developed from the transmission of light through passive birefringent sheets are generally impure and vary in appearance as a function of the viewing angle. The adapter also employs interdigitated electrodes which present a pattern of lines across the display screen. The device of Stolzer, therefore, produces color images which are not acceptable for most image display applications.
U.S. Pat. No. 4,003,081 of Hilsum et al. describes a color display system which incorporates an optically active twisted nematic liquid crystal cell that includes a separate layer of passive birefringent material to produce light of different colors. Light of the desired color is selectively transmitted through a system of neutral polarizers in response to a change in the frequency or intensity of an electric field applied to the liquid crystal cell.
U.S. Pat. No. 4,019,808 of Scheffer describes a color display system which is similar to that of Hilsum et al. Scheffer employs a twisted nematic liquid crystal cell in combination with a passive birefringent optical retardation plate or sheet to produce light of different colors. Both of the display systems described by Hilsum et al. and Scheffer produce light images which exhibit colors of an impure quality that is generally characteristic of the colors developed from passive birefringent materials. The twisted nematic liquid crystal cells are utilized in both systems to rotate the plane of polarization of light passing therethrough.
U.S. Pat. No. 4,097,128 of Matsumoto et al. and U.S. Pat. No. 4,232,948 of Shanks describe color display devices which incorporate a tunable birefringent liquid crystal cell that functions as a variable optical retarder whose birefringence is changed to develop light of various colors. In each device, the liquid crystal cell is included in an assembly of neutral polarizers which transmit light of different colors in response to a change in the birefringence of the liquid crystal cells to provide an image in full color. The change in birefringence is accomplished by varying the magnitude of a voltage which is applied to the cell. As indicated in FIG. 4 of the Matsumoto et al. patent, the maximum transmittance of light of a particular primary color which is developed by changing the birefringence of a liquid crystal cell is not accompanied by a minimum transmittance of light of the remaining primary colors. The Matsumoto et al. and Shanks devices are incapable, therefore, of providing light of pure quality. Moreover, devices using the birefringence properties of a liquid crystal material to produce light of different colors typically exhibit slow switching responses and limited viewing angles.
Switchable color filters which include optically active polarization switching devices in combination with color selective polarizing filters also have been incorporated in field sequential color display systems. An example of such a display system is disclosed in IBM Technical Disclosure Bulletin, Vol. 22, No. 5, Oct. 1979, pp. 1769-1772, "Liquid Crystal Apparatus for Converting Black and White CRT Display into Colored Display" by A. N. Brinson and A. D. Edgar who describe a switchable color filter that incorporates liquid crystal devices for converting a black and white cathode ray tube image into a full color image.
The liquid crystal device rotates by 90.degree. the polarization direction of light rays of all colors passing through the device. The ability to rotate by 90.degree. light rays of all wavelengths simplifies the task of designing a switchable color filter in that no consideration is required as respects the tuning, birefringence properties, and thickness of the liquid crystal device in specifying the orientation of and colors transmitted by the polarization axes of the polarizing filters to provide a light output of the desired color. The use of optically active liquid crystal devices has the disadvantage of requiring the application of two-frequency high voltage drive signals to switch the device at video rates to produce an image in full color.